Water Hydraulics - Wastewater Operators - page 72

 Example 2.14
Problem: If the well yield is 300 gpm and the drawdown is measured to
be 20 ft, what is the specific capacity?
Solution:
Specific Capacity = 300 ÷ 20 = 15 gpm per foot of drawdown
Specific capacity is one of the most important concepts in well
operation and testing. The calculation should be made frequently in the
monitoring of well operation. A sudden drop in specific capacity indi-
cates such problems as pump malfunction, screen plugging, or other
situations that can be serious. Such problems should be identified and
corrected as soon as possible.
2.9.2 Wet-Well hydraulics
Water pumped from a wet well by a pump set above the water sur-
face exhibits the same phenomena as the groundwater well. In opera-
tion, a slight depression of the water surface forms right at the intake
line (drawdown), but in this case it is minimal because there is free water
at the pump entrance at all times (at least there should be). The most
important consideration in wet-well operations is to ensure that the suc-
tion line is submerged far enough below the surface so air entrained by
the active movement of the water at this section is not able to enter the
pump. Because water or wastewater flow is not always constant or at the
same level, variable speed pumps are commonly used in wet-well opera-
tions, or several pumps are installed for single or combined operation.
In many cases, pumping is accomplished in an on/off mode. Control of
pump operation is in response to water level in the well. Level control
devices such as mercury switches are used to sense a high and low level
in the well and transmit the signal to pumps for action.
2.10 friCTion head loss
Materials or substances capable of flowing cannot flow freely.
Nothing flows without encountering some type of resistance. Consider
electricity, the flow of free electrons in a conductor. Whatever type of
conductor used (e.g., copper, aluminum, silver), it offers some resis-
tance. In hydraulics, the flow of water/wastewater is analogous to the
flow of electricity. Within a pipe or open channel, for instance, flowing
water (like electrons flowing in a conductor) encounters resistance. This
resistance to the flow of water is generally termed friction loss (or more
appropriately, friction head loss ).
2.10.1 flow in Pipelines
The topic of wastewater flow in pipelines (e.g., predicting flow
rates through pipes of given characteristics, calculating energy conver-
sions therein) is encountered in many applications of water/wastewater